The present invention relates to an improved composite overhead stranded conductor having optical fibers integragated in overhead power transmission line or an aerial ground wire.
Composite overhead stranded conductor having optical fibers integragated in overhead power transmission circuit.
FIG. 1 shows a cross section of a composite overhead stranded conductor of a type known in the art. As shown therein, the conductor has a central fiber unit 1 which is covered with a conductor layer 6 composed of a pluality of stranded conductors 6' such as aluminum-clad steel wires.
The optical fiber unit 1 is composed of a spacer 2 sheathed with a protective tube 5. The spacer 2, which typically is made of aluminum, is provided with a plurality of spiral grooves 3 in its periphery, each groove accommodating a fiber strand 4. The protective tube 5 is also typically made of aluminum. A cross section of one of the stranded fibers 4 housed in each spiral groove 3 is shown in FIG. 3. The fiber consists of optical fiber glass 7 coated with a cushion layer composed of a primary coat 8, typically made of a silicone resin, and a secondary coat 9, typically made of a fluorine resin.
The composite overhead stranded conductor having the construction shown above has adequate mechanical strength since the fiber strand 4 is housed within each of the spiral grooves 3 of the spacer 2 and the spacer in turn is sheathed by the protective tube 5. However, this cable design is not highly adaptable to incorporating a maximum number of coated fibers in the cable because, in order to ensure its use as a substitute for the existing overhead stranded conductors, its outside diameter must be generally equal to that of a conventional overhead stranded conductor, but the space available for accommodating coated fibers in the optical unit 1 is inadequate, being reduced by an amount corresponding to the cross-sectional area of the spacer 2. In addition, the number of grooves that can be formed in the spacer's s periphery is limited in consideration of the outside diameter of the spacer.
Assume, for example, an optical glass fiber 7 having an outside diameter of 125 microns. If a layer of silicone resin is applied over the glass fiber to form a primary coat 8 such as to provide an outside diameter of 0.4 mm, and if a secondary coat 9 typically made of a fluorine resin is subsequently applied to yield an outside diameter of 0.7 mm, then no more than give coated fibers 4 can be accommodated within each of the spiral grooves 3 formed in the periphery of the spacer 2 (which is assumed to have an outside diameter of 4.0 mm).
The composite overhead stranded conductor in use is constantly exposed to high temperatures between 100.degree. and 150.degree. C. and may be heated to nearly 300.degree. C. if a ground fault or inter-phase short circuit failure occurs due to large electric current flowing through the conductor. The transmission loss of an optical fiber is dependent on the temperature. The primary cause of this increase is microbending of the optical glass fiber which occurs when the secondary coat shrinks at elevated temperatures.